The present invention relates to a vehicle steering apparatus; for steering a vehicle by means of a steering mechanism which is not mechanically linked to a steering operating means operated by a driver.
Steering for vehicles is carried out in such a manner that operation of a steering operating means disposed inside a cabin of the vehicle, for example, rotation of a steering wheel, is transferred to a steering mechanism disposed outside the cabin for directing wheels for steering (front wheels in general).
In recent years, wide use has been made of power steering systems. A power steering system or apparatus has an actuator for assisting steering such a as hydraulic cylinder or electric motor disposed midway of the steering mechanism. Such an actuator is driven based on detection of the operational force applied to the steering wheel for steering, and operation of the steering mechanism in response to the rotation of the steering wheel is assisted by the force produced by the actuator, whereby the driver's steering effort is reduced.
However, in such a conventional vehicle steering apparatus, it is necessary to link the steering wheel or steering operating means mechanically to the steering mechanism. This results in the problem that the location of the steering wheel inside the cabin must be restricted to positions at which it can be linked to the steering mechanism disposed outside the cabin. Furthermore, even if the steering wheel is placed at a linkable position to the steering mechanism disposed outside the cabin, a complicated connecting structure is required to realize a satisfactory linking, resulting in increased vehicle weight and complication of of vehicle assembly.
In Japanese Utility Model Application Publication No. 2-29017 (1990) there is disclosed a vehicle steering apparatus which is a power steering apparatus without linkage for the purpose of eliminating the above described problems. This vehicle steering apparatus is constructed such that a steering wheel is disposed separately from a steering mechanism, while an electric motor is disposed midway of the steering mechanism as an actuator for steering, similar to an actuator for assisting steering in a power steering apparatus. In this construction, the electric motor is driven on the basis of the detect on results of an operational direction and an operation amount of the steering wheel, whereby steering is carried out in response to the operation of the steering wheel.
To the steering wheel, which is not linked mechanically to the steering mechanism, is attached a reaction force actuator provided with a motor. The reaction force actuator drives the motor on the basis of detected vehicle speed and a steering operating angle of the steering wheel, whereby a reaction force for directing the steering wheel to its neutral position, which increases; or decreases in response to vehicle speed (high or low) and the magnitude of steering operating angle (large or small) is applied to the steering wheel. In such vehicle steering apparatus, a torque applied by the operator to the steering wheel against the reaction force is detected, a motor current in the electric motor for steering is increased and decreased in response to the results detected, thereby increasing and decreasing the steering force to be produced by the electric motor. Thus, in the above described vehicle steering apparatus, steering can be carried out with the same steering feel as that of a vehicle steering apparatus wherein the steering wheel is mechanically linked to the steering mechanism.
The separate type vehicle steering apparatus constructed as described is useful for developing motor vehicle technology in the future in connection with realization of a novel steering operating means such as a lever, and pedal in place of the existing steering wheel in addition, such an apparatus could facilitate an automatic driving system using travel information, such as detection of a guide mark on the road, reception of satellite information, or the like, in addition to achieving increased flexibility for locating the steering wheel, light weight of the vehicle body, and the like.
In the separate type vehicle steering apparatus, however, when a driver loosens his (or her) hold on the steering wheel when the wheels return to their neutral positions (straight ahead traveling position) during traveling due to self-aligning torque, the torque to be applied to the steering wheel against the above-mentioned reaction force disappears, and as a result, only the reaction force directing to the neutral position is applied to the steering wheel. Hence, the steering wheel returns rapidly in this case, so that driver's steering feel becomes different from that of a conventional steering apparatus.
As a means for rotatively controlling steering wheel in case a driver loosens his (or her) hold on the steering wheel, a vehicle steering apparatus for affording a damping torque to the steering wheel for suppressing vibrations due to the road situation and the control system involved in the vehicle steering apparatus in a driver's hands free state is proposed (Japanese Patent Application Laid-Open No. 4-176781 (1992)). However, this apparatus is a means for suppressing influences derived from the road situation and the control system in case of a driver's hands free state.